1. Field of the Invention
The present invention relates to a thermal processing system capable of forming a multilayer film consisting of a plurality of films of different compositions, such as a nitride film and an oxide film.
2. Description of the Related Art
The integration size of semiconductor integrated circuits including DRAMs (dynamic random-access memories) has progressively been increasing in recent years, and various efforts have been made to improve the configuration of semiconductor integrated circuits and methods of fabricating such semiconductor integrated circuits. For example, studies have been made of the use of multilayer films, such as SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 films and SiO.sub.2 /Si.sub.3 N.sub.4 films, as capacitor insulating films for DRAMs to secure a sufficient dielectric strength for corners of trenches and to lower process temperature.
The improvement of the quality of thin films has become more important to provide integrated circuits with enhanced reliability. For example, oxygen contained in a Si.sub.3 N.sub.4 (silicon nitride) film reduces the dielectric constant and the long-term reliability of the Si.sub.3 N.sub.4 film. Therefore, a vertical thermal processing system which does not drag much air into a reactor is suitable for forming a multilayer insulating film including a Si.sub.3 N.sub.4 film.
When forming a SiO.sub.2 /Si.sub.3 N.sub.4 film on major surfaces of semiconductor wafers (hereinafter referred to simply as "wafers"), i.e., workpieces, a wafer holder holding the wafers is placed in a low-pressure CVD (LPCVD) reactor of a first vertical thermal processing system as shown in FIG. 6, and processing gases, such as NH.sub.3 and SiH.sub.2 Cl.sub.2 gas, are supplied into the LPCVD reactor to deposit a Si.sub.3 N.sub.4 (silicon nitride) film on the major surfaces of the wafers. Then, the wafer holder is taken out of the LPCVD reactor, the wafers are transferred from the wafer holder to a wafer carrier, and the wafer carrier containing the wafers is carried to a second thermal processing system as shown in FIG. 7, and a SiO.sub.2 (silicon dioxide) film is deposited over the Si.sub.3 N.sub.4 film by the second thermal processing system using a processing gas, such as tetraethoxysilane gas (abbreviated to "TEOS"). Thus, the Si.sub.3 N.sub.4 film and the SiO.sub.2 film are deposited in that order on a polysilicon layer to form a multilayer insulating film as shown in FIG. 5.
As shown in FIG. 6, the first thermal processing system 1 for forming the Si.sub.3 N.sub.4 film comprises a thermal processing unit 11, a processing gas source 13 connected through a gas supply pipe 12 and a valve V1 to the thermal processing unit 11, an exhaust system including a vacuum pump 16 connected through an exhaust pipe 14 provided with a main valve MV and a water-cooled trap 15 to the thermal processing unit 11, a pressure sensor S1 connected through a valve V2 to the exhaust pipe 14, a pressure sensor S2 connected to the gas exhaust pipe 14, and a heater 17 for heating the main valve MV and a section of the exhaust pipe 14 between the exhaust port of the thermal processing unit 11 and the water-cooled trap 15.
When forming the Si.sub.3 N.sub.4 film, NH.sub.4 Cl (ammonium chloride) power is produced as a by-product as shown by the following reaction formula. EQU 10NH.sub.3 +3SiH.sub.2 Cl.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6NH.sub.4 Cl+6H.sub.2
The deposition of NH.sub.4 Cl inside the main valve MV and the exhaust pipe 14 when the vacuum pump 16 is operated and the main valve MV is opened can be prevented if the exhaust valve 14 and the main valve MV is heated to a temperature on the order of 150.degree. C. by the heater 17. The gas exhausted from the thermal processing unit 11 is cooled by the water-cooled trap 15 to trap NH.sub.4 Cl. This prevents the reduction of the suction ability of the vacuum pump 16 and the corrosion of the component parts of the vacuum pump 16 attributable to the corrosive action of the reaction by-products and the unused source gas, as well as the reduction of the conductance of the exhaust system.
The main valve MV is closed when loading wafers W into or unloading the same from the thermal processing unit 11 to prevent causing particles by a reverse flow of the NH.sub.4 Cl powder trapped by the water-cooled trap 15 through the main valve MV and the exhaust pipe 14 into the thermal processing unit 11.
As shown in FIG. 7, the second thermal processing system 2 for forming the SiO.sub.2 film comprises a thermal processing 21, a processing gas 23 connected through a gas supply pipe 22 and a valve V1 to the thermal processing unit 21, an exhaust system including a vacuum pump 26 connected through an exhaust pipe 24 provided with a disk trap 25 and a main valve MV to the thermal processing unit 21, a pressure sensor S1 connected through a valve V2 to the exhaust pipe 24, a pressure sensor S2 connected to the gas exhaust pipe 24, and a heater 27 for heating the main valve MV and a section of the exhaust pipe 14 between the exhaust port of the thermal processing unit 21 and the main valve MV. The disk trap 25 is kept at an ordinary temperature.
When forming the SiO.sub.2 film by using TEOS, C.sub.x H.sub.y (x and y are natural numbers) is produced as a by-product as shown by the following reaction formula. EQU TEOS.fwdarw.SiO.sub.2 +C.sub.x H.sub.y +H.sub.2 O
It is difficult to trap the C.sub.x H.sub.y by simple cooling. Therefore, the conductance of the exhaust system is reduced by the disk trap 25 to trap C.sub.x H.sub.y to prevent the reduction of the suction ability of the vacuum pump 26 and the corrosion of the components of the vacuum pump 26. The disk trap 25 is disposed on the upstream side of the main valve MV, i.e., on the side of the thermal processing unit 21 with respect to the main valve MV, to prevent the deposition of C.sub.x H.sub.y inside the main valve MV having a conductance smaller than that of the exhaust pipe 14 to intercept the flow of C.sub.x H.sub.y to the main valve MV.
When forming the multilayer insulating film by using the separate thermal processing apparatuses 1 and 2, the wafers W needs to be conveyed from the thermal processing system 1 to the second thermal processing system 2. An oxide film is formed over the Si.sub.3 N.sub.4 film by natural oxidation while the wafers are being conveyed from the first thermal processing system 1 to the second thermal processing system 2. Particularly, formation of an oxide film of an uneven thickness by natural oxidation is inevitable when a door at the bottom of the thermal processing unit 11 is opened after the completion of film formation in the LPCVD reactor, because the flow of some air into the LPCVD reactor is unavoidable and the surface of the hot Si3N4 film is exposed unavoidably to air. It is difficult to remove the oxide film formed by natural oxidation completely by cleaning before the subsequent oxidation process. When transferring the wafers to a wafer holder under the thermal processing unit 21 (oxidation reactor) of the second thermal processing system 2, the further growth of the oxide film formed by natural oxidation is unavoidable because the wafers are exposed to air in an environment of a considerably high temperature under the thermal processing unit 21. Consequently, the SiO.sub.2 film is formed in the thermal processing unit 21 over the oxide film formed by natural oxidation. Thus, the multilayer insulating film including the oxide film of a low film quality reduces the reliability of integrated circuits, such as DRAMs, fabricated on the wafers.
Furthermore, the wafers are liable to be contaminated with particles because the wafers are exposed many times when taking the wafers out of the LPCVD reactor, putting the wafers in a wafer carrier, carrying the wafers and loading the wafers into the oxidation reactor for the next process. Since the multilayer insulating film is very thin and further reduction of the thickness will be necessary to meet the requirements of future DRAMs of increased integration sizes, the performance of the multilayer insulating film is deteriorated even by the contamination of the multilayer insulating film with only a few particles. Such a problem applies also to a multilayer insulating film consisting of SiO.sub.2 film and a Si.sub.3 N.sub.4 film formed on the former.
It is difficult to form multilayer insulating films, such as SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 films and SiO.sub.2 /Si.sub.3 N.sub.4 films, in satisfactory film quality by using those separate thermal processing systems, and difficulty in forming multilayer insulating films of satisfactory film quality has been an impediment to fabricating integrated circuits, such as DRAMs, having increased integration sizes.
In the semiconductor device fabricating industry, importance has been attached to the manufacturing cost of wafers required by a semiconductor chip production line and efforts have been made to reduce the manufacturing cost. However, it is reported that the curtailment of actual processing time is more important than the reduction of the manufacturing cost in recent years in view of reducing time necessary for the development of an semiconductor chip fabricating line and that the curtailment of actual processing time will become as important as the reduction of the manufacturing cost.
The inventor of the present invention considered that both the reduction of the wafer manufacturing cost and the curtailment of actual processing time can be achieved and problems attributable to the oxide film formed by natural oxidation and the contamination of wafers with particles while wafers are being transferred from one to another thermal processing system can be solved if a Si.sub.3 N.sub.4 film and a SiO.sub.2 film, which have been formed by separate thermal processing systems, are formed by a single thermal processing system, and had made studies to that effect.
If a Si.sub.3 N.sub.4 film and a SiO.sub.2 film are formed by the first thermal processing system 1 shown in FIG. 6 simply by changing the processing gases for the Si.sub.3 N.sub.4 film and the SiO.sub.2 film, respectively, whereas NH.sub.4 Cl, i.e., a by-product of the Si.sub.3 N.sub.4 film forming process, can be effectively trapped by the water-cooled trap 15, C.sub.x H.sub.y, i.e., a by-product of the SiO.sub.2 film forming process, deposits inside the main valve MV having a conductance lower than that of the exhaust pipe 14 because the water-cooled trap 15 is disposed on the downstream side of the main valve MV.
On the other hand, if a Si.sub.3 N.sub.4 film and a SiO.sub.2 film are formed by the second thermal processing system 2 shown in FIG. 7 simply by changing the processing gases for the Si.sub.3 N.sub.4 film and the SiO.sub.2 film, respectively, C.sub.x H.sub.y can be effectively trapped by the disk trap 25, and NH.sub.4 Cl also can be trapped because the temperature, i.e., an ordinary temperature, of the disk trap 25 is lower than the temperature of about 150.degree. C. of a section of the exhaust pipe 24 on the upstream side of the disk trap 25. However, since the disk trap 25 is on the upstream side of the main valve MV, it is possible that particles of NH4Cl flow reverse into the thermal processing unit 21 through the exhaust pipe 24 and adhere to the wafers even if the main valve MV is closed when loading wafers into and unloading the same from the thermal processing unit 21. Moreover, it is possible that the disk of the disk trap 25 is clogged with the trapped NH.sub.4 Cl powder and the thermal processing unit 21 may not properly be evacuated.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a thermal processing system capable of successively forming, for example, both a nitride film and an oxide film to construct a multilayer film.